The invention is generally related to an atomic force microscope probe, and more particularly to an AFM probe terminated in a chemically modified tip to amplify forces exerted on the tip due to a stronger attractive or repulsive force between the tip and the surface of a sample.
Atomic force microscopy is extensively used in microelectronics as a characterization tool. Atomic force microscopes (hereinafter AFM) are essentially surface profilometers which use sharp tips and very low forces between the tip and the sample. They also operate in a mode in which the force is attractive and the tip does not touch the surface. Typically, in the microscope, a sharp tip is placed on a flexible lever in contact with the surface. The height of the tip is detected by the tunneling microscope, and this height measurement is used in a feedback loop to move the lever up and down to keep the bending of the lever, and therefore the force on the sample to remain constant.
A prior art representation of an AFM probe is shown in FIG. 1. The AFM cantilever 102 is positioned very close to the surface 103 and is rastered by an x,y translator. During this rastering process, the deflection of the tip of the AFM is detected by means of an interferometric detector 101. With this setup, the tip can be placed directly in contact with the surface (contact mode) or can be placed at a position where the attractive forces between the tip and the surface are at a maximum (non-contact mode).
AFMs generally provide high resolution information about surface contours. Vertical movement of the sensing probe in response to a raster scanning procedure of the sensing probe across the target surface is used for determining the target surface contour. The implementation of the AFM devices is based on the interaction of forces that include atomic, electrical potential, magnetic, capacitive or chemical potential to maintain a constant probe to target surface gap or distance.
In addition to imaging surface contours, AFMs are also used to measure a variety of physical or chemical properties with detail over a range from a few Angstroms to hundreds of microns. For these applications, AFMs provide a lateral and vertical resolution that is not obtainable from any type of device. Examples of applications include imaging or measuring the contour properties of transistors, silicon chips, disk surfaces, crystal, cells and the like.
Generally, in order to provide for an optimal operation of the AFM, the scanning probe is positioned over the target surface at a distance within the same order of magnitude as molecular geometries. That is, a distance of one or two atoms, or an order of magnitude of tens of Angstroms.
Standard atomic force microscopy relies on the hard sphere interactions between a very small probe tip with the sample to be analyzed. The two common AFM modes of operation are contact and non-contact mode. In a contact mode, the tip is in direct contact with the surface while in the non-contact mode the tip remains within nanometers of the sample. The AFM tip brought to within nanometer distances of the surface and scanned in an x,y translational stage, changes in the tip position in the z-direction generates an interaction between the tip and the surface that induces a deflection which is sensed and measured by an interferometric detector. This mode of operation will be referred hereinafter as a ‘non-contact mode’.
In contrast, when the top contacts the surface and rides across the surface, the surface morphologies induce deflection of the tip which is sensed by the detector, as previously stated and which will be referred hereinafter as a ‘contact mode’. There are two main interactions that occur between the tip and the surface. One interaction is repulsive and the other is attractive. The repulsive force increases as the tip approaches the surface. This force can be explained by the hard sphere repulsions of the tip and the surface. The attractive force is due to a van der Waals type attraction between two species.
To increase the sensitivity between different species that may exist on the sample to be analyzed, probe tips have been manufactured that enhance the interaction between the tip and that material. These tips fall under the general classification of Chemically Modified AFM Tips. They are generally made in many different ways but the net result is that the chemical modification allows for better differentiation between two materials that appear to be the same under normal AFM imaging. In spite of its versatility, the amplification of a signal is achieved only on a small range of materials. Moreover, when different materials are imaged, different tip coatings are typically required. The problem is compounded by the fact that AFM tips are easily damaged due to the close proximity of the tip to the surface of the sample. Since tips are expensive, the cost of replacement becomes significant.
The goal for any imaging technique is to be able to image materials with a greater signal to noise ratio. Regarding AFM imaging, it is known that in order to increase the signal to noise ratio, chemically modified tips are preferably used to amplify the forces exerted on the tip due to stronger attractive or repulsive forces between the tip and the surface of the sample. Chemical modified tips are selected to provide certain functional groups to the end of the tip in order to accentuate the interaction between the tip and the sample. If the functional groups on the surface that is to be analyzed are polar in nature, applying a coating on a tip that is polar will be more beneficial than a non-polar functional group. If the material at the surface changes, the degree of enhancement given by the modification may or may not assist in giving an acceptable image. To attain a higher degree of image quality, quite possibly a new tip with a different tip modification would have to be needed. This becomes costly when the analyst has to routinely handle a wide range of materials.
Moreover, two problems exist with AFM and chemically modified AFM tips. One problem is that since the tips traverse the sample at a very close range, and the potential for the tip to hit the surface and damage the tip and possibly the surface is real. If the tip is significantly damaged, it needs to be replaced because the imaging quality will degrade. Another problem is directly related to the localized benefit that one achieves with chemically modified AFM tips. The tips are modified to increase the sensitivity to small amounts of materials. Basically, amplification is only achieved with materials that have similar chemical characteristics. If a new material that does not fall in that region is to be imaged, a new AFM tip needs to be created or purchased. To create the tip is not a trivial operation and the cost of the new AFM tip is significant, even if one actually exists.
A typical force for a standard AFM is an attractive van der Waals (R−6) force when it operates in a non-contact mode. The repulsive (R−12) force, on the other hand, is of the order of (R−12) when in contact mode. It has been determined that a chemical modification has the potential of contributing an additional (R−4) force to the stated interaction.
Referring now to FIG. 2, there is shown a plot of the arbitrary energy as a function of the distance between species. As two objects approach each other, an attractive R−6 potential exerts an attractive force between the objects. This attractive force increases as the distance decreases. When the distance becomes sufficiently small, a repulsive hard sphere potential with a radial dependence of R−12 exerts a repulsive force between the objects. In a real situation, the attractive force continues to draw the objects together up to a maximum force shown as the bottom of the curve. At this point, the repulsive potential starts to dampen the attractive force. If the attractive force had a stronger radial dependence, for instance, R−4, then the attractive force at the bottom of the curve will increase significantly. This increased force will generate a much larger deflection of the AFM tip which will be noted strongly by the interferometric detector. This enhanced deflection also gives a better signal to noise ratio since the signal is amplified and the noise remains the same, the noise being, of course, strongly dependent on the interferometer and not on the tip.